Chapter 19 Synapse Elimination Copyright © 2014 Elsevier Inc. All rights reserved.

Chapter 19Chapter 19

Synapse EliminationSynapse Elimination

Copyright © 2014 Elsevier Inc. All rights reserved.


FIGURE 19.1 Evidence of synapse elimination at the neuromuscular junction. (A) Drawing of a silver stainmade by J. Boeke in 1932 of the motor nerve terminals converging on two adjacent muscle fibers from the tongue muscle of a 4-day-old mouse. Each fiber is innervated by endings fromseveral different axons (see arrows). (B) Photomicrograph of several adult muscle fibers from the rat extensor digitorum longus muscle. Each adult fiber receives innervation from a single axon (arrows). Axons are impregnated with silver and stained for cholinesterase (dark oval smudge on each muscle fiber). (C) Intracellular recordings from a muscle fiber in the neonatal rat diaphragm showing multiple innervation of a skeletal muscle fiber. Gradually increasing the strength of motor nerve stimulation elicits three different postsynaptic responses in this fiber. Similar assays in adult animals give only a single postsynaptic response that cannot be fractionated by graded stimulation. The preparation was partially curarized to prevent the endplate potentials from reaching threshold. Figures adapted from (A) Boeke (1932); (B) Gorio, Marini, and Zanoni (1983); (C) Redfern (1970).


FIGURE 19.2 In the developing nervous system synapse number is increasing while axonal convergence is falling. The total number of preganglionic synapses on ganglion neurons in the rat submandibular ganglion increases in early postnatal life during the period of synapse elimination. Camera lucida drawings of clusters of ganglion neurons at birth and in adult animals that were treated with zinc iodide osmium. This reagent selectively fills terminal axons and synaptic boutons with a dense black precipitate. The number of synaptic boutons increases (compare the amount of synaptic boutons (arrows) at birth and on adult ganglion cells), and the number of preganglionic axons innervating each ganglion cell, identified by electrophysiological measurements, decreases (see the number of electrophysiological steps (arrow), each representing an input, at early postnatal life and in adult ganglion neurons, see lower panels). Thus axons that are not removed create new synapses to more than compensate for the loss of synapses from other axons. From Lichtman (1977).


FIGURE 19.3 Changes in fan-in and fan-out at developing circuits. In neonatal vertebrates, neurons (A, green and B, red) project their axons to fan out to many target cells (each labeled ab). These nascent connections are typically weak and mediated only by a small number of synaptic contacts (triangles from the green and red axons). Axons undergo several structural rearrangements before reaching adulthood. First, axons disconnect from many target cells reducing their fan-out or divergence. Second, this rearrangement leads to less fan-in or convergence. Third, at the same time as axons disconnect from some target cells they are strengthening their connection with other target cells by increasing the number of synapses at their remaining targets.


FIGURE 19.4 Profound changes in axonal arbors and synapses at birth. (A) Left, The axonal arbor of a motor axon’s motor unit from a postnatal day 13 mouse. The neuromuscular junctions innervated by this axon are highlighted (yellow), and the rest of the neuromuscular junctions are shown in gray. Right, The branching arbor of a motor unit in the same muscle at postnatal day 1. At this young age the axon arbor is dramatically more complicated, and it innervates manyfold more neuromuscular junctions (yellow). B, The image on the left shows a serial electron microscopy 3D reconstruction of a single neuromuscular junction at birth that was then traced to show that 11 different axons converged on the site. On the right, each of the 11 axons that innervate this neuromuscular junction at birth are separately displayed, showing that they are quite intermingled and none occupies a majority of the neuromuscular junction.


FIGURE 19.5 Time-lapse imaging shows how axonal branches are lost during synapse elimination at the neuromuscular junction. Two neuromuscular junctions (NMJ1 and NMJ2) were viewed in vivo on postnatal days 7, 8, and 9 in a transgenic mouse that expresses YFP in its motor axons (green). The acetylcholine receptors at the muscle fiber membrane are labeled with rhodamine-tagged α-bungarotoxin (red). On day 7, NMJ1 is multiply innervated, whereas NMJ2 is singly innervated by a branch of one of the axons that innervates NMJ1. One day later (postnatal day 8), one of the motor axons innervatingNMJ1 becomes thinner all the way back to its branch point. When viewed on postnatal day 9, this thin branch is no longer connected to NMJ1 and now ends in a bulb-shaped swelling that is called a retraction bulb. Modified from Keller-Peck et al. (2001).


FIGURE 19.6 Retracting axons shed material as they disappear. The inset shows a surface rendering of a serial electron microscopy reconstruction of a retraction bulb (green, arrowhead) during synapse elimination at a developing neuromuscular junction. The electron micrograph shows that the retracting axon contains clusters of vesicles and mitochondria. In the vicinity of the bulb but not connected are small spherical axonal fragments (axosomes, arrow) that are engulfed by glial cells (i.e., Schwann cells (SC), darker cytoplasm). Scale bars, 1mm. From Bishop, Misgeld, Walsh, Gan, and Lichtman (2004).


FIGURE 19.7 A diagram showing differences between the vertebrate and invertebrate synaptic circuits. (A) Invertebrates have small numbers (for example, sometimes just one motor neuron) of identifiable neurons innervating small numbers of identifiable target cells (e.g., sometimes just one muscle fiber), whereas in vertebrates, pools of similar neurons innervating targets contain hundreds or thousands of similar postsynaptic cells (B). The redundancy in the vertebrate nervous system allows a neuron to diverge (fan out) and innervate many equally appropriate target cells (in the neuromuscular system, this is called a motor unit) (C). The homogeneity of innervating neurons allows multiple axons to converge (fan in) on each target cell (D). As a result, vertebrate circuits contain a substantial amount of fan-in and fan-out (E), at least initially. Synapse elimination’s purpose may be to transform such a set of redundant circuits into multiple unique ones by trimming away the multiple innervation of target cells (F). The result of the widespread loss of synapses is the generation of thousands of nonredundant circuits from an initially much less specific innervation pattern. From Lichtman and Colman (2000).


FIGURE 19.8 Segregation of synaptic territory as axons compete during synapse elimination. Double transgenic mice expressing YFP and CFP in different motor axons were used to observe axons’ nerve terminal interactions. At birth is shown two competing nerve terminals, one expressing CFP (blue) and the other YFP (green) at this neuromuscular junction, which are intermingled. By the second postnatal week, however, YFP and CFP axon terminals are generally completely segregated from each other in multiply innervated junctions. Red is fluorescently tagged α-bungarotoxin that binds to AChRs. Modified from Gan and Lichtman (1998).


FIGURE 19.9 In vivo imaging shows takeover of synaptic territory by the remaining axon during the period of synapse elimination. Neuromuscular junctions in transgenic mice that express YFP (yellow) and CFP (blue) were imaged multiple times during early postnatal life. (A–E) views of one neuromuscular junction between P8 and P15. The CFP axon takes over occupancy of the postsynaptic sites (labeled red) in the upper parts of the junction that were formerly innervated by the YFP axon. The YFP labeled axon withdrew until only a retraction bulb remained (E, asterisk). At P12, a process of the CFP axon had begun to invade the territory of the YFP axon (D, circle and arrow in inset). (F–J) Although the CFP axon (blue and insets) has greater terminal area (~70%) at the first view, it progressively withdraws from the junction (arrows). Its retraction bulb can be seen in (I) and (J) (asterisks). Scale bars = 10 μm. Insets show the blue axons. From Walsh and Lichtman (2003).


FIGURE 19.10 Asynchronous synapse elimination among the branches of one axon. Using transgenic animals that express YFP in a small subset of motor axons, it was possible to monitor the behavior of multiple branches of the same axon. This diagram shows the typical result for an axon in the midst of the synapse elimination process. Represented in red are the AChRs on each muscle fiber (represented as gray tubes). This motor axon (black) has won the competition on the bottom muscle fiber and occupies the entire receptor plaque. The same axon has lost the competition for the adjacent muscle fiber, where only a retraction bulb remains. The three other neuromuscular junctions that this axon innervates are still undergoing competition. In one case a small portion of AChRs is being innervated by this axon. It is likely that this synapse will be eliminated. On the other two muscle fibers, each axon terminal occupies 50% of the AChRs. This indicates that its fate is not yet determined in these junctions. Adapted from Lichtman and Colman (2000).


FIGURE 19.11 Axonal fate at individual neuromuscular junctions is related to the identity of the competing axons. Using transgenic animals that express YFP (yellow) and CFP (blue) in a small subset of motor axons, it was possible to reconstruct two motor units in which several neuromuscular junctions are innervated by the exact same two axons, one expressing YFP and the other CFP. At these coinnervated junctions, it appears that the same outcome is occurring at each. In this case, the YFP axon always occupied more territory than the CFP axon. Note also that the axon calibers of the YFP axon branches are thicker than CFP branches to the same junctions. Thus synapse elimination seems to be biased such that when the same two axons compete, the fate is the same at each coinnervated neuromuscular junction. Image from Kasthuri and Lichtman (2003).


FIGURE 19.12 Postsynaptic cells that don’t fire action potentials don’t undergo synapse elimination. Photomicrographs of multiply and singly innervated snake muscle fibers. The singly innervated neuromuscular junctions are on twitch muscle fibers that have voltage-sensitive sodium channels. The multiply innervated neuromuscular junctions are found on tonic muscle fibers that do not have regenerative potentials. Labeling of different axons with different colors was accomplished by activity-dependent fluorescent labeling of axon terminals. From Lichtman, Wilkinson, and Rich (1985).


FIGURE 19.13 An experimental test of the role of postsynaptic activity in synapse elimination. If axons converging on the same postsynaptic cell were competing based on their efficacy in activating the postsynaptic cell, then locally blocking synaptic transmission at one axon’s synaptic site should lead to its elimination and the maintenance of the other unblocked inputs. In this test, focal blockade of neuromuscular transmission was accomplished by applying saturating doses of α-bungarotoxin with a fine glass pipette (upper panels, red) to a neuromuscular junction in a living mouse. The AChR sites (middle panels) and the nerve terminals (lower panels) were viewed at the time of blockade and several times over the next few weeks. Following focal blockade (left panels), progressive loss of the nerve terminal staining and AChRs (arrows) was observed in regions previously saturated with α-bungarotoxin. However, when the entire neuromuscular junction was blocked (right panels), there was no loss. These results argue that active postsynaptic synaptic sites can cause the disassembly of inactive sites. Adapted from Balice-Gordon and Lichtman (1994).


FIGURE 19.14 Putative mechanism for a postsynaptic role in synapse elimination. Postsynaptic receptor activation may elicit two opposing signals within a muscle fiber. One consequence of receptor activation is a “punishment” signal (here designated as red arrows) that causes destabilization of synaptic sites. Receptor activation may also generate a “protective” signal (here designated as blue clouds) that locally prohibits the punishment signals from destabilizing synapses in the vicinity of where receptor activation recently occurred. (A) When all the receptors are activated synchronously, as might occur before birth when motor neurons are electrically coupled (Personius & Balice-Gordon, 2001), there is no synaptic destabilization (due to protective blue clouds everywhere). (B) However, later in development, when two inputs are activated asynchronously, the active synaptic sites at any time point are protected (i.e., the synaptic sites beneath the active green axon are protected by a local blue cloud), whereas the asynchronously activated synapses (i.e., the synaptic sites under the pink axon) are not protected. (C) This asynchrony allows the more powerful input to destabilize the weaker one. This destabilization can lead to nerve and postsynaptic disappearance and/or nerve withdrawal, followed quickly by takeover of its former synaptic sites by the remaining axon, which would restabilize the site. (D) If all synaptic sites are inactive, as might occur with α-bungarotoxin application that inactivates nicotinic AChRs, then there are no blue clouds to protect synaptic sites, but also no red arrows to destabilize them. Thus synaptically silent and synchronously active synaptic sites (see panel A) do not undergo synapse elimination. Idea adapted from Jennings (1994).


FIGURE 19.15 Synapse elimination in the visual system. (A) Ocular dominance columns of the neonatal monkey primary visual cortex in layer IVC, revealed by injecting [3H]-proline into the vitreous of one eye. Light stripes (columns) represent sites containing the anterograde transported 3H-amino acid from the injected eye. Dark regions are occupied by axons driven by the other eye. (B) Monocular deprivation by lid suture of one eye (2 weeks after birth for a period of 18months) resulted in the shrinkage of the columns representing the deprived eye (dark stripes) and an expansion of the columns of the nondeprived eye (light stripes). (C) Aschematic representation of ocular dominance column development represents the way in which a gradual segregation ocular dominance column could lead to the end of the critical period and progressively more modest effects of monocular deprivation as development ensues. (Top) At birth the afferents from the two eyes (red and green ovals) overlap completely in layer IV, and thus each eye is capable of maintaining inputs everywhere. At this young age, monocular deprivation would allow the nondeprived eye to remain in all parts of layer IV so that the entire cortex would be dominated by the red inputs. (Middle) In nondeprived animals, the two sets of afferents become progressively more segregated with age, meaning that by three weeks there would be regions of layer IV that are exclusively driven by the red or, as shown, green afferents. Once an eye’s inputs are removed from a territory, it can no longer reoccupy that territory when the other eye is silenced. Hence monocular deprivation (that begins at three weeks) will spare a small strip of the inactive eye’s territory (in this case the green regions). (Bottom) Once segregation is complete then monocular deprivation has no effect and the critical period is over. (D) A remarkable example showing how interactions between two eyes can cause segregation was found in frogs in which a third eye is implanted (at the tadpole stage) next to one of the eyes and projects with the native eye to the same optic tectum (ordinarily each frog tectum is monocular). After injection of H3-proline into the normal eye, one optic tectum of a three-eyed frog shows dark and light bands strikingly similar to the ocular dominance columns observed in monkeys (D). A–C adapted from Hubel et al. (1977); D from Constantine-Paton and Law (1978).


FIGURE 19.16 Synapse elimination in the lateral geniculate nucleus of the thalamus. (A) In the ferret lateral geniculate nucleus, receptive fields of geniculate neurons are larger and much more diffuse at one month of age than those receptive fields observed in adult neurons. Red regions represent the receptive field map of geniculate neurons that correspond to areas excited by bright stimuli. Note that red areas become smaller as development proceeds. The shrinkage in the receptive field is likely to result from the elimination of the convergence of multiple retinal afferents onto each geniculate neuron (B). At P12 in mouse, multiple retinogeniculate axons are recruited as stimulation intensities to the bundle of axons is increased (see also Fig. 19.1C). At P17 there are fewer steps, and after P28, only one or two inputs innervate each geniculate neuron (i.e., no steps in the evoked-synaptic currents are seen even though optic nerve stimulation is increased). (A) Adapted from Tavazoie and Reid (2000). (B) Adapted from Chen and Regehr (2000).


FIGURE 19.17 Immature retinal ganglion cells show correlated patterns of activity. (A) Using an array of extracellular recordings, rhythmic bursts of action potentials (indicated by vertical lines) are synchronized between neighboring retinal ganglion cells before eye opening in ferret. (B) Action potential bursts expanded in time scale show that each burst corresponds to ten or more action potentials. (C) Using calcium indicators it has been possible to observe waves of neuronal activity in immature retinal ganglion cells (pseudo-colored image). In this example, a wave of neuronal activity propagates through the retina. Pseudocolored cells indicating the temporal firing pattern of retinal ganglion neurons; cells in green fire before yellow ones, and lastly red cells. (A, B) from Meister, Wong, Baylor, and Shatz (1991). (C) from Wong (1999).


FIGURE 19.18 Synapse elimination and axonal pruning of climbing fibers in the neonatal period. (A) Recordings from Purkinje cells while olivocerebellar axons are stimulated in the inferior olive show functional evidence of synapse elimination. At birth, each Purkinje cell is innervated by several different climbing fibers, and as the strength of stimulation is increased, additional inputs are recruited (compare with Figs. 19.1C and 19.15B). The average number of climbing fibers innervating each cerebellar Purkinje cell, in the rat, decreases gradually as the animal matures until most are singly innervated. (B, C) Reconstructions of the trajectory of single neonatal (B) and adult (C) olivocerebellar climbing fiber axons. Both neonatal and adult axons terminate in several separate lobules in the hemisphere. However, neonatal olivocerebellar axons have many more branches than those in adult axons and presumably innervate many more Purkinje cells than those in adult animals. After pruning is complete, each axon gives rise to ~7 climbing fibers that each singly innervates a different Purkinje cell. (A) Adapted from Hashimoto and Kano (2003). (B, C) Modified from Sugihara (2005).

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